Focussing on three sympatric species, the Great hornbill (Buceros bicornis), the Wreathed hornbill (Aceros undulatus), and the Oriental Pied hornbill (Anthracoceros albirostris), in a lowland semi-evergreen forest in Arunachal Pradesh (Pakhui National Park), this study (1997-2000) attempted to answer broad questions about hornbill biology. In addition, the dispersal syndromes of tree species were evaluated, and their patterns of flowering and fruiting examined in the light of existing hypotheses about the evolution of fruiting patterns.

A small proportion of trees were found to be wind-dispersed, with bimodal flowering and fruiting peaks that coincided with the relatively drier periods in the year. Of the 64% of tree species that were animal-dispersed, 57% were bird-dispersed including many species of Lauraceae and Meliaceae. Flowering of bird-dispersed species occurred throughout the year, while the fruiting peak was unimodal (May-July), and a trough (fruit scarcity) between September and January. Null model analysis of fruiting schedules of 7 bird-dispersed species showed their fruiting patterns to be significantly segregated, indicating the possible effect of competition for dispersers. Although, climatic factors may be the main force dictating the timing of fruiting, fruiting of bird-dispersed species may also be partly shaped by biotic factors. Hornbills consumed 80 fruit species, dispersing about one-fourth of the tree species in the area, including seeds of large fruits of Meliaceae, Myristicaceae, and Lauraceae that probably rely exclusively on hornbills for dispersal. Hornbills are relatively high quality dispersers, due to benign seed processing, seed viability, relatively long gut-retention times and because seeds are deposited away from parent fruiting trees. However, seed deposition patterns below nest and roost trees are spatially clumped, compromising dispersal quality due to increased seed predation and density dependent mortality. Hornbills, possibly play a more efficient role as seed dispersers at perch trees, since seed clumping and seed predation rates are lower than that under nest or roost trees.

Breeding of all three hornbill species commenced in the dry hot period (March-April) and continued into the monsoon (June-August) contrary to earlier reports. The nesting cycle was longer for the Great hornbill (110-129 days, N = 8), and the Wreathed hornbill (120-140 days, N = 9) than that of the Oriental Pied hornbill (93-97 days, n = 2). Great hornbill females emerged out of the nest cavity after 88 to 109 days, whereas females of the other two species emerged with the chicks. The timing of breeding coincided with peak ripe fruit availability.

Fruits dominated (> 90%) in the diet of all three species throughout the breeding season, though animal matter increased marginally in the post-hatching period. Though a wide variety of non-fig fruit species were eaten, ten species contributed over 90% of the non-fig fruit diet in both the breeding and non-breeding season. Communal roost sites were on isolated deciduous trees either in successional grassland habitats or on cliff faces along rivers and perennial streams. Communal roosting occurred during both the breeding and non-breeding seasons, with larger flocks seen during the latter period, especially for the Wreathed hornbill. Communal roosting appears to be mainly related to food finding and sharing of information about food sources.

Hornbills nested in cavities of live trees of five genera with 83% of 36 nests being in Tetrameles nudiflora. Differences between hornbill species in nest site choice were largely determined by cavity size. Although nesting success was high (80%), 51% of nest trees were inactive at the end of four years, mainly due to human-related disturbances. Potential limitation of nest sites and current rates of loss of nesting habitat are matters of concern for hornbill conservation.

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